Table of contents

SQUIDs are a class of very sensitive superconducting measuring devices, whose operation is based on flux quantization and the Josephson effects. After a brief summary of these effects the principles of operation of single and double Josephson junction SQUIDs are dealt with in some detail, including noise processes and estimates of the ultimate sensitivity attainable. Applications of these devices in a wide variety of fields including precise electrical measurement, thermometry, magnetic properties, relativity physics, geophysical measurement and biomagnetism are dealt with.

A transversely excited, CO₂, waveguide laser of relatively simple construction is described. Operating at a gas pressure of one atmosphere, it produces 10 mu m wavelength pulses with peak powers of over 1 kW.

It is shown that a general treatment of the properties of vibrating sample magnetometer pickup coils, leads to the possibility of induced signals at twice the frequency of sample vibration. Whereas the fundamental signal A( omega ₀) is proportional to the vibrational amplitude a₀ and the quantity delta h_z/ delta z in a 'one-dimensional' system, the doubled signal A(2 omega ₀) is proportional to a₀² and delta ²h_z/ delta z². Possible applications of this effect to two sample magnetometers and magnetic anisotropy measurements are discussed.

A new technique is described for the continuous and rapid measurement of the thermo-electric power of substances as a function of either temperature or pressure. The circuitry involves the use of operational amplifiers-extremely efficient and versatile devices. The overall accuracy of the instrument constructed out of this circuitry is within 1%. The performance of the system is demonstrated by giving the experimental curve of thermopower against temperature in the case of ytterbium.

A 'pile-up' inspector is described which removes the distortion due to the arrival of more than one photon per detection cycle in a single photon counting experiment. The apparatus described furnishes a strobe pulse to the time to amplitude converter (TAC) if and only if no more than one photon has been detected during the cycling time of the TAC. While eliminating errors due to detection of multiple photon events, the inspector also permits considerably increased photon counting rates in a single photon counting experiment.

A triac isolator circuit has been developed to minimize damage to conductors and insulation during AC electric strength tests. The polarity of the test voltage at the instant of breakdown is indicated.

A new flowmeter under development at NEL is described, consisting of an orifice plate with a spring-loaded disc moving in an annular expansion. The differential pressure across the annular orifice is directly proportional to the flowrate. Tests on a first prototype of 105 mm diameter have given encouraging results.

A variable temperature liquid helium cooling unit for use with ESR spectrometers is described. The unit is based on the continuous flow principle and the advantages of this method, particularly with regard to ESR experiments are explained. The unit is part of a complete cryogenic system which includes a flexible gas cooled helium transfer tube, a gas flow pump and controller and an automatic temperature controller. Sample temperatures between 3.7K and 300K can be obtained readily and the stability varies between +or-0.02K and +or-0.08K over this range. The total helium consumption of the system varies between 1.01 h^-1 at 4.2K and 0.51 h^-1 at 10K and decreases further with increased temperature. This compact cooling unit is well suited to other experiments with restrictive geometries.

Some general factors affecting the stability of low-level oscillators are considered and a design is given for an active circuit to drive a low Q tank circuit with a dissipation below 10^-9 W. The circuit is insensitive to supply voltage variations, and contributes less than 1 part per million per degree, to the drift in frequency with temperature. The results of tests of its performance driving a magnetic thermometer used below 1K are described.

The temperature dependence of gauge factor and magnetoresistance for two types of foil strain gauge commonly used in magnetostriction measurements has been investigated. Although the temperature variation of gauge factor for either type does not change significantly from one batch to another, the magnetoresistance in Ni-Cr alloy gauges is less predictable. The relative merits of the two types are discussed.

The chopper was designed for dynamic electron optical experiments where it is necessary to provide pulses of high quality with respect to both width and energy spread. The chopping action relies on the optical properties of a microwave cavity; these are exploited such that the time dependent space focusing causes a small circular aperture on the axis of rotational symmetry to transmit strongly for a single band of phase angles in each cycle of the excitation. The theoretical treatment is oriented towards computer display, and is novel in that it follows the progress of individual electron packets throughout the system. In contrast to conventional chopping and bunching theory, it is possible to analyse with ease the pulsing properties of a multicavity device.

A magnetic mass filter of the half-period Wien filter type is described. Formulae and graphs necessary for the design of half-period filters are given and several important aspects of the design are described in detail. The design of a molecular beam detector, incorporating a half-period filter, is discussed. In this application, the small, straight line magnet is advantageous. Many important independent requirements are fulfilled simultaneously by the relatively simple design, which utilizes the mass 'discrimination' in the ion source for good mass suppression despite the use of high transmission and low resolution.

Details are given of a spherical mirror open resonator suitable for use in a radio refractometer operating at 36 GHz. The resonator, constructed of silver plated brass spherical reflectors spaced with fused silica tubing with 90% of the wall cut away has a loaded Q of 11000, an open area to sampling volume ratio of 2.7 cm^-1 and a temperature coefficient better than 0.5 N units/ degrees C. The small sampling volume of 3.7 cm³ allows a much higher spatial resolution than the conventional X-band sampling cavity.

A coil suspended in a magnetic field can behave as a capacitor of surprisingly high value. A DC motor falls into this category and with appropriate circuitry can be used to measure the moment of inertia of an object attached to its shaft. Possible uses are suggested, and the effects of friction in the system are discussed.

A technique using Lissajous figures is described which may be used for measuring complex AC conductivity at very low frequencies. The apparatus is simple and the technique can measure nonlinear properties. An addition to the basic apparatus allows an automatic computer analysis.

A reliable flashlamp pumped dye laser is described which takes advantage of a pre-pulse and of a simmering mode of triggering. Increased power output, long life of the electronic components, and excellent reproducibility in time and intensity of laser output are obtained.

The theory and construction of a microwave cavity with crossed-grid end walls for the measurement of the Hall effect are described. This cavity avoids the background signal from the cavity walls that obscures the measurements in low mobility materials.

A non destructive method and an apparatus are described which can be used to measure the optical absorption coefficient in thin semiconductor layers. By using a long optical path length, absorption coefficients of the order of 0.05 cm^-1 can be measured.

Describes an absorption spectrophotometric arrangement with a wavenumber linear scanning of the spectra and a double beam system built (on a kinematic mounting) before a small aperture ratio spectrograph (F/35-F=3.4 m). The two symmetrical light beams are temporally separated by rotating splitters and are received alternately on the same measuring attachment. The electronic treatment of the signals uses the 'sample and hold' technique and gives directly the integrated absorbance of the transition studied and consequently its oscillator strength. The double beam system can be used with cells up to 50 cm in length, which can be placed in ovens or cryostats. Results are presented for the forbidden transition ¹A_1g to ³B_1u of liquid benzene at 340 nm. An oscillator strength of 8*10^-8 was obtained for this transition.

The influence of an AC magnetic field on the image of a scanning electron microscope is derived by simple theory. The magnitude of the perturbation using several field intensities is evaluated from measurements carried out on a JSM-50A scanning electron microscope under different imaging conditions using the emissive mode of operation. The performance of a JSM-50A scanning microscope equipped with an automatically compensating device, recently developed for transmission microscopes, is tested under several AC field intensities. It is demonstrated that the influence of an AC magnetic field of power frequency on the image quality is eliminated to a high degree and no difference in image quality exists between micrographs of the same object taken in a low field of about 0.1 A m^-1 and in fields as high as 1 A m^-1.

Describes a method of automatically coding various images by density lines using a television system on-line with a minicomputer. Essentially only the spatial coordinates of the points where a variable threshold level is equal to the video signal are digitized and stored. Therefore, all these points for a given threshold level on a television raster give one isodensity line. The accuracy of the isodensity lines depends chiefly on the type of television device used. Results are related by using the plumbicon tube XQ 1023 R (Philips).

An interferometer based on the Kosters prism is described. The interferometer is highly stable due to the symmetry between the two interfering beams and a very high accuracy is achieved by the incorporation of a high sensitivity fringe detection system. The sensitivity is better than 10^-4 of the measuring wavelength, but the present accuracy is limited to 10^-3 by environmental and system noise. The interferometer is best suited for the analysis of thin transparent films and the determination of the thickness of reflecting films.

The use of cascaded channel plates as position dependent particle counters is described. A one- and two-dimensional dissection technique, based on the spread of charge between channel plate and collector, is used. The one-dimensional detector is developed for use in atomic scattering experiments. The system is tested by measuring the relative, total differential scattering cross section of electrons by helium between 1 degrees and 3.4 degrees .

An accurate and reproducible determination of the magnification in point-projection or electron-shadow microscopy is obtainable by using as a reference the objective lens current required for infinite magnification. The magnification at any other objective lens current is then determined as a function of the difference between that current and the current at infinite magnification. It is generally a simple matter to choose a reproducible current difference. A replica of a crossed grating with line spacing of 0.463 mu m was used as a test specimen in experiments with a converted Siemens Elmiskop 1. The relationship, magnification=8720/n, where 12.8 n mA is the current difference, was found to hold to better than 3%.